The present invention is directed to a method for producing semiconductor material layers composed of amorphous silicon-germanium alloys, particularly for use in constructing solar cells. The layers are produced by deposition from a vapor phase, whereby gases containing gaseous hydrogenated or fluorinated silicon and germanium compounds are excited in a gas discharge plasma (low-pressure plasma) in a reactor between electrodes by means of inductive and if necessary capacitive radio-frequency coupling and are deposited as solid, amorphous silicon-germanium layers a-Si.sub.x Ge.sub.1-x : Y (Y=H, F) on electrodes or substrates attached thereto. The present invention also is directed to an apparatus for the implementation of the method.
U.S. Pat. No. 4,292,343, for example, discloses methods and apparatus for the deposition of pure, amorphous silicon layers (a-Si:H). FIG. 1 of U.S. Pat. No. 4,292,343 describes a capacitative, and FIG. 2 therein describes an inductive rf-glow discharge reactor arrangement. PIN solar cells, based on amorphous silicon, are produced on substrates that are later used as contacts.
The glow discharge technique also provides the possibility of depositing a plurality of PIN cells, that have different energy gaps, on top of one another. Such double or triple tandem cells allows the entire solar spectrum to be more effectively utilized; this provides a greater cell efficiency. To lower the energy gap, germanium can be added to amorphous silicon. The resultant cell thereby becomes more red-sensitive. However, the density of states of the silicon-germanium layers increases greatly, during typical depositions, with an increasing germanium content. The result is that germanium-rich layers, if used for thin-film tandem solar cells (having an energy gap below 1.5 eV), exhibit poor semiconductor properties.
The general belief is that this phenomenon is due to the fact that the ionization and bonding energy of a hydrogen atom in GeH.sub.4 is lower at 10.5 eV than that in SiH.sub.4 at 11.4 eV. Accordingly, the dangling bonds in an amorphous SiGe:H material are not saturated with hydrogen to the same degree as in an amorphous Si:H material. When silicon and germanium are in juxtaposition in an amorphous structure, then silicon is thereby preferentially saturated by hydrogen atoms. Therefore, it is difficult to uniformly saturate silicon atoms and germanium atoms with hydrogen through a simultaneous deposition of silicon and germanium with the same radio frequency source, dc source, or light source.
One method for attempting to improve the saturation of the dangling-bonds in the silicon-germanium alloys is to dilute the silane-germane mixtures with hydrogen. This, however, fails in germanium rich layers that do not have a sufficient quality to be satisfactorily used in thin-film tandem solar cells.